STRUCTURAL AND METAMORPHIC EVOLUTION OF VARISCAN BASEMENT ROCKS OF THE ALPS IN COMPARISON WITH OTHER PORTIONS OF THE EUROPEAN VARISCAN BELT

The tectonic evolution of the Southern Variscan belt is still poorly understood if compared with that of the central European Variscan belt. That is due to intense late-Variscan to Alpine re-working, in particular in the Alpine axial zone. The Variscan rocks of the external domains of the Alps are instead little affected by Alpine re-equilibration. For this reason, these domains are eligible for constraining the evolution of the Southern Variscan belt, since they well preserve the Variscan record. This thesis is focused on the Variscan basement rocks in the Argentera-Mercantour Massif (Helvetic domain), Orobic basement (Southalpine domain), and Maures-Tanneron Massif, which is a benchmark for the Variscan belt just outside the Alpine area. The aim is to provide new constraints on the Variscan and Alpine tectonic evolution of the external domains of the Alps and in the Alpine foreland. This study required structural mapping (1:10.000 to 1:100 scale) integrated by microstructural, chemical, and mineral-chemical analyses. Metamorphic conditions that affected the Variscan basements are estimated in terms of pressure and temperature and constrained in time by relative and absolute ages. By doing so, several stages of the Variscan and Alpine tectonics are addressed, from early Cambrian to Tertiary times, and discussed in light of the geodynamic and paleogeographic reference models for both the Variscan and Alpine belts. The results refer to four main tectonic stages of Southern Variscan belt, from the subduction of the Variscan ocean to the Alpine collision: First, we constrain the structural and metamorphic evolution of remnants of early Cambrian oceanic lithosphere in the Argentera-Mercantour massif. These rocks experienced prehnite-pumpellyite to eclogite facies conditions (T = 610 – 660°C, P = 1.90 – 2.30 GPa) before exhumation in Carboniferous times. A cold Variscan subduction zone is thus envisaged for the first time in the Helvetic domain of the Alps. Second, we refine the tectonic evolution of the external domains of the Southern Variscan belt through a comparison between the western Maures-Tanneron Massif and Orobic basement. The western Maures-Tanneron and the Orobic basement retain a similar litho-stratigraphy and tectonic evolution from early-Cambrian to Permian times, with peak metamorphic conditions (T = 550 – 650°C, P = 0.6 – 0.8 GPa) attained during the Variscan collision. A correlation between these two domains, today in opposite position with respect to the subduction complexes of the Southern Variscan belt, is thus proposed. Third, we decipher the signal of the Permian-Triassic lithospheric thinning on both sides of the Alpine Tethys Ocean, in the Argentera-Mercanotur Massif and Orobic basement. Here, swarms of calc-alkaline and alkaline lamprophyres emplaced at shallow crustal levels in the exhumed Variscan basement. Since the production of mantle-derived melts at the end of the Variscan cycle required forced extension that cannot result from the sole gravitational collapse of the Variscan belt, the emplacement of these mafic magmas is interpreted as marking the beginning of the Alpine cycle in the Alpine portion of the Southern Variscan belt. Fourth, we quantify the Alpine tectono-metamorphic evolution retained by the Variscan basement rocks in the external domains of the Alps. It follows that similar Alpine peak conditions (T = 420 – 450°C and P = 0.15 – 0.40 GPa) were reached in the Helvetic domain and Southern Alps, on both sides of the Alpine orogenic wedge.